In many conventional electric power steering systems (EPS), a brushless DC motor is used as an EPS motor. The brushless DC motor generates no brush sliding noises It is of course possible to use AC motors as EPS motors. It is however generally recognized that the brushless DC motor is most suitable for the electric power steering systems, because it is not only compact in size but has large torque, small loss and small cogging torque.
The brushless DC motor is a magnet-rotor synchronous motor, which is driven by an inverter. The inverter is conventionally driven by a 120°-energization method, a 180°-energization method or a sine-wave energization method. Here, the brushless DC motor is not limited to a type of rectangular-wave energization method. The EPS motor is generally driven by a sine-wave modulation method to reduce vibrations, which a driver will sense, because the sine-wave modulation method reduces torque ripples, noises and vibrations.
As other modulation methods, a two-phase modulation method and a trapezoid-wave modulation method are known. In the two-phase modulation method, the inter-phase voltage waveform is maintained in a sine waveform by fixing one phase potential and modulating the remaining two phase potentials in sine-wave. In the trapezoid-wave modulation method, harmonic waves of odd-number orders are added to a sine-wave phase voltage. These two-phase modulation method and the trapezoid-wave modulation method are advantageous in producing more torque than by the sine-wave modulation method.
In one two-phase modulation method, that is a conventional π/3 fixing method, each phase voltage is fixed to a high potential level and a low potential level for an electric angle of 60° (π/3) alternately by sequentially turning on switching elements of an inverter for a predetermined period while maintaining an inter-phase voltage. This method reduces the switching loss of the inverter.
U.S. Pat. No. 4,847,743 (JP 2,577,738) proposes another two-phase modulation method, that is, a 2π/3 fixing method, in which each phase voltage is fixed to a high potential level and a low potential level for an electric angle of 120° (2π/3). This method also reduces the switching loss of the inverter. It also proposes to apply three phase-voltages to the motor by stopping the two-phase modulation method, when the amplitude of the phase voltage is small.
U.S. Pat. No. 7,053,587 (JP 2005-229676A) proposes that a two-phase modulation method is implemented when a phase current is large, and a three-phase modulation method is effected when the phase current is small. This method also reduces the switching loss.
It is desired that the brushless DC motor used as the EPS motor generates vibration and noise that will be transferred to a driver through a steering shaft as little as possible. In this respect, although the two-phase modulation method produces large torque, it generates large vibration and noise due to distortion of waveform and hence is not suited for use as the EPS motor. Although the three-phase sine-wave modulation method is most suited to reduce vibration and noise, the motor becomes large in size to ensure large torque and hence has poor response characteristics due to increased rotor inertial mass.